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Versions: 00 01 02 03 04 05 06 07 RFC 3032

Network Working Group                                      Eric C. Rosen
Internet Draft                                             Yakov Rekhter
Expiration Date: March 2000                                Daniel Tappan
                                                          Dino Farinacci
                                                            Guy Fedorkow
                                                     Cisco Systems, Inc.

                                                                 Tony Li
                                                  Juniper Networks, Inc.

                                                              Alex Conta
                                                     Lucent Technologies

                                                          September 1999

                       MPLS Label Stack Encoding

                  draft-ietf-mpls-label-encaps-07.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   Drafts.

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   and may be updated, replaced, or obsoleted by other documents at any
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   "Multi-Protocol Label Switching (MPLS)" [1,2] requires a set of
   procedures for augmenting network layer packets with "label stacks",
   thereby turning them into "labeled packets".  Routers which support
   MPLS are known as "Label Switching Routers", or "LSRs".  In order to

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   transmit a labeled packet on a particular data link, an LSR must
   support an encoding technique which, given a label stack and a
   network layer packet, produces a labeled packet.  This document
   specifies the encoding to be used by an LSR in order to transmit
   labeled packets on PPP data links, on LAN data links, and possibly on
   other data links as well.  On some data links, the label at the top
   of the stack may be encoded in a different manner, but the techniques
   described here MUST be used to encode the remainder of the label
   stack.  This document also specifies rules and procedures for
   processing the various fields of the label stack encoding.

Table of Contents

    1      Introduction  ...........................................   3
    1.1    Specification of Requirements  ..........................   3
    2      The Label Stack  ........................................   3
    2.1    Encoding the Label Stack  ...............................   3
    2.2    Determining the Network Layer Protocol  .................   6
    2.3    Generating ICMP Messages for Labeled IP Packets  ........   7
    2.3.1  Tunneling through a Transit Routing Domain  .............   7
    2.3.2  Tunneling Private Addresses through a Public Backbone  ..   8
    2.4    Processing the Time to Live Field  ......................   9
    2.4.1  Definitions  ............................................   9
    2.4.2  Protocol-independent rules  .............................   9
    2.4.3  IP-dependent rules  .....................................  10
    2.4.4  Translating Between Different Encapsulations  ...........  10
    3      Fragmentation and Path MTU Discovery  ...................  11
    3.1    Terminology  ............................................  12
    3.2    Maximum Initially Labeled IP Datagram Size  .............  13
    3.3    When are Labeled IP Datagrams Too Big?  .................  14
    3.4    Processing Labeled IPv4 Datagrams which are Too Big  ....  14
    3.5    Processing Labeled IPv6 Datagrams which are Too Big  ....  15
    3.6    Implications with respect to Path MTU Discovery  ........  16
    4      Transporting Labeled Packets over PPP  ..................  17
    4.1    Introduction  ...........................................  17
    4.2    A PPP Network Control Protocol for MPLS  ................  18
    4.3    Sending Labeled Packets  ................................  19
    4.4    Label Switching Control Protocol Configuration Options  .  19
    5      Transporting Labeled Packets over LAN Media  ............  19
    6      IANA Considerations  ....................................  20
    7      Security Considerations  ................................  20
    8      Intellectual Property  ..................................  20
    9      Authors' Addresses  .....................................  21
   10      References  .............................................  22

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1. Introduction

   "Multi-Protocol Label Switching (MPLS)" [1,2] requires a set of
   procedures for augmenting network layer packets with "label stacks",
   thereby turning them into "labeled packets".  Routers which support
   MPLS are known as "Label Switching Routers", or "LSRs".  In order to
   transmit a labeled packet on a particular data link, an LSR must
   support an encoding technique which, given a label stack and a
   network layer packet, produces a labeled packet.

   This document specifies the encoding to be used by an LSR in order to
   transmit labeled packets on PPP data links and on LAN data links.
   The specified encoding may also be useful for other data links as
   well.

   This document also specifies rules and procedures for processing the
   various fields of the label stack encoding.  Since MPLS is
   independent of any particular network layer protocol, the majority of
   such procedures are also protocol-independent.  A few, however, do
   differ for different protocols.  In this document, we specify the
   protocol-independent procedures, and we specify the protocol-
   dependent procedures for IPv4 and IPv6.

   LSRs that are implemented on certain switching devices (such as ATM
   switches) may use different encoding techniques for encoding the top
   one or two entries of the label stack.  When the label stack has
   additional entries, however, the encoding technique described in this
   document MUST be used for the additional label stack entries.

1.1. Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [3].

2. The Label Stack

2.1. Encoding the Label Stack

   The label stack is represented as a sequence of "label stack
   entries".  Each label stack entry is represented by 4 octets.  This
   is shown in Figure 1.

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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Label
   |                Label                  | Exp |S|       TTL     | Stack
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Entry

                       Label:  Label Value, 20 bits
                       Exp:    Experimental Use, 3 bits
                       S:      Bottom of Stack, 1 bit
                       TTL:    Time to Live, 8 bits

                                 Figure 1

   The label stack entries appear AFTER the data link layer headers, but
   BEFORE any network layer headers.  The top of the label stack appears
   earliest in the packet, and the bottom appears latest.  The network
   layer packet immediately follows the label stack entry which has the
   S bit set.

   Each label stack entry is broken down into the following fields:

      1. Bottom of Stack (S)

         This bit is set to one for the last entry in the label stack
         (i.e., for the bottom of the stack), and zero for all other
         label stack entries.

      2. Time to Live (TTL)

         This eight-bit field is used to encode a time-to-live value.
         The processing of this field is described in section 2.4.

      3. Experimental Use

         This three-bit field is reserved for experimental use.

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      4. Label Value

         This 20-bit field carries the actual value of the Label.

         When a labeled packet is received, the label value at the top
         of the stack is looked up.  As a result of a successful lookup
         one learns:

            (a) the next hop to which the packet is to be forwarded;

            (b) the operation to be performed on the label stack before
                forwarding; this operation may be to replace the top
                label stack entry with another, or to pop an entry off
                the label stack, or to replace the top label stack entry
                and then to push one or more additional entries on the
                label stack.

         In addition to learning the next hop and the label stack
         operation, one may also learn the outgoing data link
         encapsulation, and possibly other information which is needed
         in order to properly forward the packet.

         There are several reserved label values:

              i. A value of 0 represents the "IPv4 Explicit NULL Label".
                 This label value is only legal when it is the sole
                 label stack entry.  It indicates that the label stack
                 must be popped, and the forwarding of the packet must
                 then be based on the IPv4 header.

             ii. A value of 1 represents the "Router Alert Label".  This
                 label value is legal anywhere in the label stack except
                 at the bottom.  When a received packet contains this
                 label value at the top of the label stack, it is
                 delivered to a local software module for processing.
                 The actual forwarding of the packet is determined by
                 the label beneath it in the stack.  However, if the
                 packet is forwarded further, the Router Alert Label
                 should be pushed back onto the label stack before
                 forwarding.  The use of this label is analogous to the
                 use of the "Router Alert Option" in IP packets [6].
                 Since this label cannot occur at the bottom of the
                 stack, it is not associated with a particular network
                 layer protocol.

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            iii. A value of 2 represents the "IPv6 Explicit NULL Label".
                 This label value is only legal when it is the sole
                 label stack entry.  It indicates that the label stack
                 must be popped, and the forwarding of the packet must
                 then be based on the IPv6 header.

             iv. A value of 3 represents the "Implicit NULL Label".
                 This is a label that an LSR may assign and distribute,
                 but which never actually appears in the encapsulation.
                 When an LSR would otherwise replace the label at the
                 top of the stack with a new label, but the new label is
                 "Implicit NULL", the LSR will pop the stack instead of
                 doing the replacement.  Although this value may never
                 appear in the encapsulation, it needs to be specified
                 in the Label Distribution Protocol, so a value is
                 reserved.

              v. Values 4-15 are reserved.

2.2. Determining the Network Layer Protocol

   When the last label is popped from a packet's label stack (resulting
   in the stack being emptied), further processing of the packet is
   based on the packet's network layer header.  The LSR which pops the
   last label off the stack must therefore be able to identify the
   packet's network layer protocol.  However, the label stack does not
   contain any field which explicitly identifies the network layer
   protocol.  This means that the identity of the network layer protocol
   must be inferable from the value of the label which is popped from
   the bottom of the stack, possibly along with the contents of the
   network layer header itself.

   Therefore, when the first label is pushed onto a network layer
   packet, either the label must be one which is used ONLY for packets
   of a particular network layer, or the label must be one which is used
   ONLY for a specified set of network layer protocols, where packets of
   the specified network layers can be distinguished by inspection of
   the network layer header.  Furthermore, whenever that label is
   replaced by another label value during a packet's transit, the new
   value must also be one which meets the same criteria.  If these
   conditions are not met, the LSR which pops the last label off a
   packet will not be able to identify the packet's network layer
   protocol.

   Adherence to these conditions does not necessarily enable
   intermediate nodes to identify a packet's network layer protocol.
   Under ordinary conditions, this is not necessary, but there are error

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   conditions under which it is desirable.  For instance, if an
   intermediate LSR determines that a labeled packet is undeliverable,
   it may be desirable for that LSR to generate error messages which are
   specific to the packet's network layer.  The only means the
   intermediate LSR has for identifying the network layer is inspection
   of the top label and the network layer header.  So if intermediate
   nodes are to be able to generate protocol-specific error messages for
   labeled packets, all labels in the stack must meet the criteria
   specified above for labels which appear at the bottom of the stack.

   If a packet cannot be forwarded for some reason (e.g., it exceeds the
   data link MTU), and either its network layer protocol cannot be
   identified, or there are no specified protocol-dependent rules for
   handling the error condition, then the packet MUST be silently
   discarded.

2.3. Generating ICMP Messages for Labeled IP Packets

   Section 2.4 and section 3 discuss situations in which it is desirable
   to generate ICMP messages for labeled IP packets.  In order for a
   particular LSR to be able to generate an ICMP packet and have that
   packet sent to the source of the IP packet, two conditions must hold:

      1. it must be possible for that LSR to determine that a particular
         labeled packet is an IP packet;

      2. it must be possible for that LSR to route to the packet's IP
         source address.

   Condition 1 is discussed in section 2.2.  The following two
   subsections discuss condition 2.  However, there will be some cases
   in which condition 2 does not hold at all, and in these cases it will
   not be possible to generate the ICMP message.

2.3.1. Tunneling through a Transit Routing Domain

   Suppose one is using MPLS to "tunnel" through a transit routing
   domain, where the external routes are not leaked into the domain's
   interior routers.  For example, the interior routers may be running
   OSPF, and may only know how to reach destinations within that OSPF
   domain.  The domain might contain several Autonomous System Border
   Routers (ASBRs), which talk BGP to each other.  However, in this
   example the routes from BGP are not distributed into OSPF, and the
   LSRs which are not ASBRs do not run BGP.

   In this example, only an ASBR will know how to route to the source of

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   some arbitrary packet.  If an interior router needs to send an ICMP
   message to the source of an IP packet, it will not know how to route
   the ICMP message.

   One solution is to have one or more of the ASBRs inject "default"
   into the IGP.  (N.B.: this does NOT require that there be a "default"
   carried by BGP.) This would then ensure that any unlabeled packet
   which must leave the domain (such as an ICMP packet) gets sent to a
   router which has full routing information.  The routers with full
   routing information will label the packets before sending them back
   through the transit domain, so the use of default routing within the
   transit domain does not cause any loops.

   This solution only works for packets which have globally unique
   addresses, and for networks in which all the ASBRs have complete
   routing information.  The next subsection describes a solution which
   works when these conditions do not hold.

2.3.2. Tunneling Private Addresses through a Public Backbone

   In some cases where MPLS is used to tunnel through a routing domain,
   it may not be possible to route to the source address of a fragmented
   packet at all.  This would be the case, for example, if the IP
   addresses carried in the packet were private (i.e., not globally
   unique) addresses, and MPLS were being used to tunnel those packets
   through a public backbone.  Default routing to an ASBR will not work
   in this environment.

   In this environment, in order to send an ICMP message to the source
   of a packet, one can copy the label stack from the original packet to
   the ICMP message, and then label switch the ICMP message.  This will
   cause the message to proceed in the direction of the original
   packet's destination, rather than its source.  Unless the message is
   label switched all the way to the destination host, it will end up,
   unlabeled, in a router which does know how to route to the source of
   original packet, at which point the message will be sent in the
   proper direction.

   This technique can be very useful if the ICMP message is a "Time
   Exceeded" message or a "Destination Unreachable because fragmentation
   needed and DF set" message.

   When copying the label stack from the original packet to the ICMP
   message, the label values must be copied exactly, but the TTL values
   in the label stack should be set to the TTL value that is placed in
   the IP header of the ICMP message.  This TTL value should be long
   enough to allow the circuitous route that the ICMP message will need

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   to follow.

   Note that if a packet's TTL expiration is due to the presence of a
   routing loop, then if this technique is used, the ICMP message may
   loop as well. Since an ICMP message is  never sent as a result of
   receiving an ICMP message, and since many implementations throttle
   the rate at which ICMP messages can be generated, this is not
   expected to pose a problem.

2.4. Processing the Time to Live Field

2.4.1. Definitions

   The "incoming TTL" of a labeled packet is defined to be the value of
   the TTL field of the top label stack entry when the packet is
   received.

   The "outgoing TTL" of a labeled packet is defined to be the larger
   of:

      (a) one less than the incoming TTL,
      (b) zero.

2.4.2. Protocol-independent rules

   If the outgoing TTL of a labeled packet is 0, then the labeled packet
   MUST NOT be further forwarded; nor may the label stack be stripped
   off and the packet forwarded as an unlabeled packet.  The packet's
   lifetime in the network is considered to have expired.

   Depending on the label value in the label stack entry, the packet MAY
   be simply discarded, or it may be passed to the appropriate
   "ordinary" network layer for error processing (e.g., for the
   generation of an ICMP error message, see section 2.3).

   When a labeled packet is forwarded, the TTL field of the label stack
   entry at the top of the label stack MUST be set to the outgoing TTL
   value.

   Note that the outgoing TTL value is a function solely of the incoming
   TTL value, and is independent of whether any labels are pushed or
   popped before forwarding.  There is no significance to the value of
   the TTL field in any label stack entry which is not at the top of the
   stack.

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2.4.3. IP-dependent rules

   We define the "IP TTL" field to be the value of the IPv4 TTL field,
   or the value of the IPv6 Hop Limit field, whichever is applicable.

   When an IP packet is first labeled, the TTL field of the label stack
   entry MUST BE set to the value of the IP TTL field.  (If the IP TTL
   field needs to be decremented, as part of the IP processing, it is
   assumed that this has already been done.)

   When a label is popped, and the resulting label stack is empty, then
   the value of the IP TTL field SHOULD BE replaced with the outgoing
   TTL value, as defined above.  In IPv4 this also requires modification
   of the IP header checksum.

   It is recognized that there may be situations where a network
   administration prefers to decrement the IPv4 TTL by one as it
   traverses an MPLS domain, instead of decrementing the IPv4 TTL by the
   number of LSP hops within the domain.

2.4.4. Translating Between Different Encapsulations

   Sometimes an LSR may receive a labeled packet over, e.g., a label
   switching controlled ATM (LC-ATM) interface [10], and may need to
   send it out over a PPP or LAN link.  Then the incoming packet will
   not be received using the encapsulation specified in this document,
   but the outgoing packet will be sent using the encapsulation
   specified in this document.

   In this case, the value of the "incoming TTL" is determined by the
   procedures used for carrying labeled packets on, e.g., LC-ATM
   interfaces.  TTL processing then proceeds as described above.

   Sometimes an LSR may receive a labeled packet over a PPP or a LAN
   link, and may need to send it out, say, an LC-ATM interface.  Then
   the incoming packet will be received using the encapsulation
   specified in this document, but the outgoing packet will not be sent
   using the encapsulation specified in this document.  In this case,
   the procedure for carrying the value of the "outgoing TTL" is
   determined by the procedures used for carrying labeled packets on,
   e.g., LC-ATM interfaces.

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3. Fragmentation and Path MTU Discovery

   Just as it is possible to receive an unlabeled IP datagram which is
   too large to be transmitted on its output link, it is possible to
   receive a labeled packet which is too large to be transmitted on its
   output link.

   It is also possible that a received packet (labeled or unlabeled)
   which was originally small enough to be transmitted on that link
   becomes too large by virtue of having one or more additional labels
   pushed onto its label stack.  In label switching, a packet may grow
   in size if additional labels get pushed on.  Thus if one receives a
   labeled packet with a 1500-byte frame payload, and pushes on an
   additional label, one needs to forward it as frame with a 1504-byte
   payload.

   This section specifies the rules for processing labeled packets which
   are "too large".  In particular, it provides rules which ensure that
   hosts implementing Path MTU Discovery [5], and hosts using IPv6
   [8,9], will be able to generate IP datagrams that do not need
   fragmentation, even if those datagrams get labeled as they traverse
   the network.

   In general, IPv4 hosts which do not implement Path MTU Discovery [5]
   send IP datagrams which contain no more than 576 bytes.  Since the
   MTUs in use on most data links today are 1500 bytes or more, the
   probability that such datagrams will need to get fragmented, even if
   they get labeled, is very small.

   Some hosts that do not implement Path MTU Discovery [5] will generate
   IP datagrams containing 1500 bytes, as long as the IP Source and
   Destination addresses are on the same subnet.  These datagrams will
   not pass through routers, and hence will not get fragmented.

   Unfortunately, some hosts will generate IP datagrams containing 1500
   bytes, as long the IP Source and Destination addresses have the same
   classful network number.  This is the one case in which there is any
   risk of fragmentation when such datagrams get labeled.  (Even so,
   fragmentation is not likely unless the packet must traverse an
   ethernet of some sort between the time it first gets labeled and the
   time it gets unlabeled.)

   This document specifies procedures which allow one to configure the
   network so that large datagrams from hosts which do not implement
   Path MTU Discovery get fragmented just once, when they are first
   labeled.  These procedures make it possible (assuming suitable
   configuration) to avoid any need to fragment packets which have
   already been labeled.

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3.1. Terminology

   With respect to a particular data link, we can use the following
   terms:

     - Frame Payload:

       The contents of a data link frame, excluding any data link layer
       headers or trailers (e.g., MAC headers, LLC headers, 802.1Q
       headers, PPP header, frame check sequences, etc.).

       When a frame is carrying an unlabeled IP datagram, the Frame
       Payload is just the IP datagram itself.  When a frame is carrying
       a labeled IP datagram, the Frame Payload consists of the label
       stack entries and the IP datagram.

     - Conventional Maximum Frame Payload Size:

       The maximum Frame Payload size allowed by data link standards.
       For example, the Conventional Maximum Frame Payload Size for
       ethernet is 1500 bytes.

     - True Maximum Frame Payload Size:

       The maximum size frame payload which can be sent and received
       properly by the interface hardware attached to the data link.

       On ethernet and 802.3 networks, it is believed that the True
       Maximum Frame Payload Size is 4-8 bytes larger than the
       Conventional Maximum Frame Payload Size (as long as neither an
       802.1Q header nor an 802.1p header is present, and as long as
       neither can be added by a switch or bridge while a packet is in
       transit to its next hop).  For example, it is believed that most
       ethernet equipment could correctly send and receive packets
       carrying a payload of 1504 or perhaps even 1508 bytes, at least,
       as long as the ethernet header does not have an 802.1Q or 802.1p
       field.

       On PPP links, the True Maximum Frame Payload Size may be
       virtually unbounded.

     - Effective Maximum Frame Payload Size for Labeled Packets:

       This is either the Conventional Maximum Frame Payload Size or the
       True Maximum Frame Payload Size, depending on the capabilities of
       the equipment on the data link and the size of the data link
       header being used.

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     - Initially Labeled IP Datagram:

       Suppose that an unlabeled IP datagram is received at a particular
       LSR, and that the the LSR pushes on a label before forwarding the
       datagram.  Such a datagram will be called an Initially Labeled IP
       Datagram at that LSR.

     - Previously Labeled IP Datagram:

       An IP datagram which had already been labeled before it was
       received by a particular LSR.

3.2. Maximum Initially Labeled IP Datagram Size

   Every LSR which is capable of

      (a) receiving an unlabeled IP datagram,
      (b) adding a label stack to the datagram, and
      (c) forwarding the resulting labeled packet,

   SHOULD support a configuration parameter known as the "Maximum
   Initially Labeled IP Datagram Size", which can be set to a non-
   negative value.

   If this configuration parameter is set to zero, it has no effect.

   If it is set to a positive value, it is used in the following way.
   If:
      (a) an unlabeled IP datagram is received, and
      (b) that datagram does not have the DF bit set in its IP header,
          and
      (c) that datagram needs to be labeled before being forwarded, and
      (d) the size of the datagram (before labeling) exceeds the value
          of the parameter,
   then
      (a) the datagram must be broken into fragments, each of whose size
          is no greater than the value of the parameter, and
      (b) each fragment must be labeled and then forwarded.

   For example, if this configuration parameter is set to a value of
   1488, then any unlabeled IP datagram containing more than 1488 bytes
   will be fragmented before being labeled.  Each fragment will be
   capable of being carried on a 1500-byte data link, without further
   fragmentation, even if as many as three labels are pushed onto its
   label stack.

   In other words, setting this parameter to a non-zero value allows one

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   to eliminate all fragmentation of Previously Labeled IP Datagrams,
   but it may cause some unnecessary fragmentation of Initially Labeled
   IP Datagrams.

   Note that the setting of this parameter does not affect the
   processing of IP datagrams that have the DF bit set; hence the result
   of Path MTU discovery is unaffected by the setting of this parameter.

3.3. When are Labeled IP Datagrams Too Big?

   A labeled IP datagram whose size exceeds the Conventional Maximum
   Frame Payload Size of the data link over which it is to be forwarded
   MAY be considered to be "too big".

   A labeled IP datagram whose size exceeds the True Maximum Frame
   Payload Size of the data link over which it is to be forwarded MUST
   be considered to be "too big".

   A labeled IP datagram which is not "too big" MUST be transmitted
   without fragmentation.

3.4. Processing Labeled IPv4 Datagrams which are Too Big

   If a labeled IPv4 datagram is "too big", and the DF bit is not set in
   its IP header, then the LSR MAY silently discard the datagram.

   Note that discarding such datagrams is a sensible procedure only if
   the "Maximum Initially Labeled IP Datagram Size" is set to a non-zero
   value in every LSR in the network which is capable of adding a label
   stack to an unlabeled IP datagram.

   If the LSR chooses not to discard a labeled IPv4 datagram which is
   too big, or if the DF bit is set in that datagram, then it MUST
   execute the following algorithm:

      1. Strip off the label stack entries to obtain the IP datagram.

      2. Let N be the number of bytes in the label stack (i.e, 4 times
         the number of label stack entries).

      3. If the IP datagram does NOT have the "Don't Fragment" bit set
         in its IP header:

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            a. convert it into fragments, each of which MUST be at least
               N bytes less than the Effective Maximum Frame Payload
               Size.

            b. Prepend each fragment with the same label header that
               would have been on the original datagram had
               fragmentation not been necessary.

            c. Forward the fragments

      4. If the IP datagram has the "Don't Fragment" bit set in its IP
         header:

            a. the datagram MUST NOT be forwarded

            b. Create an ICMP Destination Unreachable Message:

                    i. set its Code field [4] to "Fragmentation Required
                       and DF Set",

                   ii. set its Next-Hop MTU field [5] to the difference
                       between the Effective Maximum Frame Payload Size
                       and the value of N

            c. If possible, transmit the ICMP Destination Unreachable
               Message to the source of the of the discarded datagram.

3.5. Processing Labeled IPv6 Datagrams which are Too Big

   To process a labeled IPv6 datagram which is too big, an LSR MUST
   execute the following algorithm:

      1. Strip off the label stack entries to obtain the IP datagram.

      2. Let N be the number of bytes in the label stack (i.e, 4 times
         the number of label stack entries).

      3. If the IP datagram contains more than 1280 bytes (not counting
         the label stack entries), or if it does not contain a fragment
         header, then:

            a. Create an ICMP Packet Too Big Message, and set its Next-
               Hop MTU field to the difference between the Effective
               Maximum Frame Payload Size and the value of N

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            b. If possible, transmit the ICMP Packet Too Big Message to
               the source of the datagram.

            c. discard the labeled IPv6 datagram.

      4. If the IP datagram is not larger than 1280 octets, and it
         contains a fragment header, then

            a. Convert it into fragments, each of which MUST be at least
               N bytes less than the Effective Maximum Frame Payload
               Size.

            b. Prepend each fragment with the same label header that
               would have been on the original datagram had
               fragmentation not been necessary.

            c. Forward the fragments.

         Reassembly of the fragments will be done at the destination
         host.

3.6. Implications with respect to Path MTU Discovery

   The procedures described above for handling datagrams which have the
   DF bit set, but which are "too large", have an impact on the Path MTU
   Discovery procedures of RFC 1191 [5].  Hosts which implement these
   procedures will discover an MTU which is small enough to allow n
   labels to be pushed on the datagrams, without need for fragmentation,
   where n is the number of labels that actually get pushed on along the
   path currently in use.

   In other words, datagrams from hosts that use Path MTU Discovery will
   never need to be fragmented due to the need to put on a label header,
   or to add new labels to an existing label header.  (Also, datagrams
   from hosts that use Path MTU Discovery generally have the DF bit set,
   and so will never get fragmented anyway.)

   Note that Path MTU Discovery will only work properly if, at the point
   where a labeled IP Datagram's fragmentation needs to occur, it is
   possible to cause an ICMP Destination Unreachable message to be
   routed to the packet's source address.  See section 2.3.

   If it is not possible to forward an ICMP message from within an MPLS
   "tunnel" to a packet's source address, but the network configuration
   makes it possible for the LSR at the transmitting end of the tunnel
   to receive packets that must go through the tunnel, but are too large
   to pass through the tunnel unfragmented, then:

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     - The LSR at the transmitting end of the tunnel MUST be able to
       determine the MTU of the tunnel as a whole.  It MAY do this by
       sending packets through the tunnel to the tunnel's receiving
       endpoint, and performing Path MTU Discovery with those packets.

     - Any time the transmitting endpoint of the tunnel needs to send a
       packet into the tunnel, and that packet has the DF bit set, and
       it exceeds the tunnel MTU, the transmitting endpoint of the
       tunnel MUST send the ICMP Destination Unreachable message to the
       source, with code "Fragmentation Required and DF Set", and the
       Next-Hop MTU Field set as described above.

4. Transporting Labeled Packets over PPP

   The Point-to-Point Protocol (PPP) [7] provides a standard method for
   transporting multi-protocol datagrams over point-to-point links.  PPP
   defines an extensible Link Control Protocol, and proposes a family of
   Network Control Protocols for establishing and configuring different
   network-layer protocols.

   This section defines the Network Control Protocol for establishing
   and configuring label Switching over PPP.

4.1. Introduction

   PPP has three main components:

      1. A method for encapsulating multi-protocol datagrams.

      2. A Link Control Protocol (LCP) for establishing, configuring,
         and testing the data-link connection.

      3. A family of Network Control Protocols for establishing and
         configuring different network-layer protocols.

   In order to establish communications over a point-to-point link, each
   end of the PPP link must first send LCP packets to configure and test
   the data link.  After the link has been established and optional
   facilities have been negotiated as needed by the LCP, PPP must send
   "MPLS Control Protocol" packets to enable the transmission of labeled
   packets.  Once the "MPLS Control Protocol" has reached the Opened
   state, labeled packets can be sent over the link.

   The link will remain configured for communications until explicit LCP
   or MPLS Control Protocol packets close the link down, or until some
   external event occurs (an inactivity timer expires or network

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   administrator intervention).

4.2. A PPP Network Control Protocol for MPLS

   The MPLS Control Protocol (MPLSCP) is responsible for enabling and
   disabling the use of label switching on a PPP link.  It uses the same
   packet exchange mechanism as the Link Control Protocol (LCP).  MPLSCP
   packets may not be exchanged until PPP has reached the Network-Layer
   Protocol phase.  MPLSCP packets received before this phase is reached
   should be silently discarded.

   The MPLS Control Protocol is exactly the same as the Link Control
   Protocol [7] with the following exceptions:

      1. Frame Modifications

         The packet may utilize any modifications to the basic frame
         format which have been negotiated during the Link Establishment
         phase.

      2. Data Link Layer Protocol Field

         Exactly one MPLSCP packet is encapsulated in the PPP
         Information field, where the PPP Protocol field indicates type
         hex 8281 (MPLS).

      3. Code field

         Only Codes 1 through 7 (Configure-Request, Configure-Ack,
         Configure-Nak, Configure-Reject, Terminate-Request, Terminate-
         Ack and Code-Reject) are used.  Other Codes should be treated
         as unrecognized and should result in Code-Rejects.

      4. Timeouts

         MPLSCP packets may not be exchanged until PPP has reached the
         Network-Layer Protocol phase.  An implementation should be
         prepared to wait for Authentication and Link Quality
         Determination to finish before timing out waiting for a
         Configure-Ack or other response.  It is suggested that an
         implementation give up only after user intervention or a
         configurable amount of time.

      5. Configuration Option Types

         None.

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4.3. Sending Labeled Packets

   Before any labeled packets may be communicated, PPP must reach the
   Network-Layer Protocol phase, and the MPLS Control Protocol must
   reach the Opened state.

   Exactly one labeled packet is encapsulated in the PPP Information
   field, where the PPP Protocol field indicates either type hex 0281
   (MPLS Unicast) or type hex 0283 (MPLS Multicast).  The maximum length
   of a labeled packet transmitted over a PPP link is the same as the
   maximum length of the Information field of a PPP encapsulated packet.

   The format of the Information field itself is as defined in section
   2.

   Note that two codepoints are defined for labeled packets; one for
   multicast and one for unicast.  Once the MPLSCP has reached the
   Opened state, both label switched multicasts and label switched
   unicasts can be sent over the PPP link.

4.4. Label Switching Control Protocol Configuration Options

   There are no configuration options.

5. Transporting Labeled Packets over LAN Media

   Exactly one labeled packet is carried in each frame.

   The label stack entries immediately precede the network layer header,
   and follow any data link layer headers, including, e.g., any 802.1Q
   headers that may exist.

   The ethertype value 8847 hex is used to indicate that a frame is
   carrying an MPLS unicast packet.

   The ethertype value 8848 hex is used to indicate that a frame is
   carrying an MPLS multicast packet.

   These ethertype values can be used with either the ethernet
   encapsulation or the 802.3 LLC/SNAP encapsulation to carry labeled
   packets.  The procedure for choosing which of these two
   encapsulations to use is beyond the scope of this document.

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6. IANA Considerations

   Label values 0-15 inclusive have special meaning, as specified in
   this document, or as further assigned by IANA.

   In this document, label values 0-3 are specified in section 2.1.

   Label values 4-15 may be assigned by IANA, based on IETF Consensus.

7. Security Considerations

   The MPLS encapsulation that is specified herein does not raise any
   security issues that are not already present in either the MPLS
   architecture [1] or in the architecture of the network layer protocol
   contained within the encapsulation.

   There are two security considerations inherited from the MPLS
   architecture which may be pointed out here:

     - Some routers may implement security procedures which depend on
       the network layer header being in a fixed place relative to the
       data link layer header.  These procedures will not work when the
       MPLS encapsulation is used, because that encapsulation is of a
       variable size.

     - An MPLS label has its meaning by virtue of an agreement between
       the LSR that puts the label in the label stack (the "label
       writer") , and the LSR that interprets that label (the "label
       reader").  However, the label stack does not provide any means of
       determining who the label writer was for any particular label.
       If labeled packets are accepted from untrusted sources, the
       result may be that packets are routed in an illegitimate manner.

8. Intellectual Property

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.

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9. Authors' Addresses

   Eric C. Rosen
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   E-mail: erosen@cisco.com

   Dan Tappan
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   E-mail: tappan@cisco.com

   Dino Farinacci
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134
   E-mail: dino@cisco.com

   Yakov Rekhter
   Cisco Systems, Inc.
   170 Tasman Drive
   San Jose, CA, 95134
   E-mail: yakov@cisco.com

   Guy Fedorkow
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA, 01824
   E-mail: fedorkow@cisco.com

   Tony Li
   Juniper Networks
   385 Ravendale Dr.
   Mountain View, CA, 94043
   E-mail: tli@juniper.net

   Alex Conta
   Lucent Technologies
   300 Baker Avenue
   Concord, MA, 01742
   E-mail: aconta@lucent.com

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10. References

   [1] Rosen, E., Viswanathan, A., and Callon, R., "Multiprotocol Label
   Switching Architecture", Work in Progress, April 1999.

   [2] Callon, R., Doolan, P., Feldman, N., Fredette, A., Swallow, G.,
   Viswanathan, A., "A Framework for Multiprotocol Label Switching",
   Work in Progress, November 1997.

   [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", RFC 2119, BCP 14, March 1997.

   [4] Postel, J., "Internet Control Message Protocol", RFC 792,
   September 1981.

   [5] Mogul, J. and Deering S., "Path MTU Discovery", RFC 1191,
   November 1990.

   [6] Katz, D., "IP Router Alert Option", RFC 2113, February 1997.

   [7] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
   1661, STD 51, July 1994.

   [8] Conta, A. and Deering, S., "Internet Control Message Protocol
   (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification",
   RFC 1885, December 1995.

   [9] McCann, J., Deering, S. and Mogul, J., "Path MTU Discovery for IP
   version 6", RFC 1981, August 1996.

   [10] Davie, B., Lawrence, J., McCloghrie, K., Rekhter, Y., Rosen, E.
   and Swallow G., "MPLS Using LDP and ATM VC Switching", Work in
   Progress, April 1999.

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